1. Field of the Invention
The present invention relates to a thin-film transistor (TFT)-type liquid-crystal display device and more particularly to the TFT-type liquid-crystal display device having an improved capacitor section and to a method for manufacturing the same.
2. Description of the Related Art
An active matrix-type liquid-crystal display device employing a TFT as a switching device is so configured that a TFT array substrate on which the TFTs and picture element electrodes are mounted in a matrix form is placed, through a liquid-crystal, facing a color filter substrate on which a light-intercepting film (so called a “black matrix”), a coloring layer and a common electrode are formed.
FIG. 34 is a schematic equivalent circuit diagram showing a conventional liquid-crystal display device described above. In FIG. 34, address wiring 11 constitutes a scanning line driven by an address wiring driver (not shown) connected to a gate terminal 301, data wiring 12 constitutes a signal line driven by a data wiring driver (not shown) connected to a drain terminal 302, a reference numeral 103 denotes a thin-film transistor section with its gate connected to the address wiring 11 and with its drain connected to the data wiring 12 and a reference numeral 6 denotes a transparent electrode which is connected to the thin-film transistor section 103 and is formed with a transparent conductive film composed of ITO (indium tin oxide) or the like. An area encircled by a broken line shows a TFT array substrate 100 and a color filter substrate 200 and these two substrates facing each other are mounted through the liquid-crystal.
To a source of the thin-film transistor section 103 are in parallel connected a capacitor section 105 and a liquid-crystal capacitor section 310. The capacitor section 105 is a capacitive device composed of the transparent electrode 6 and of an accumulative capacitive electrode disposed through an insulating film below the transparent electrode 6. The liquid-crystal capacitor section 310 is a capacitive device composed of the transparent electrode 6 and of another electrode (not shown) facing on the color filter substrate 200 disposed through the liquid-crystal. This causes a voltage inputted from a common potential inputting terminal 303 mounted on the TFT array substrate 100 to be supplied by a transfer pad (not shown) through the transfer device 304 to the facing electrode (not shown).
In the liquid-crystal display device in FIG. 34, selecting pulses are sequentially applied by the address wiring driver to the address wiring 11. When the selecting pulse is applied to any of address wiring 11, all the thin-film transistor sections 103 connected to the address wiring 11 become conductive only for the period while the selecting pulse is being applied. At this point, the transparent electrode 6 connected to the source of the thin-film transistor section 103 is charged to a signal voltage which has been applied to the data wiring 12. After that, when non-selecting pulse is applied to the address wiring 11, though the thin-film transistor section 103 which has been in a conductive state is then turned OFF, the transparent electrode 6 still continues holding a charged voltage. The voltage being held, when any corresponding thin-film transistor section 103 becomes conductive, becomes rewritten by a subsequent voltage.
In order for a liquid-crystal display device using the TFT array substrate 100 to perform its displaying having high quality, it is necessary for the picture element electrode to be able to hold the charged voltage until its subsequent writing is complete. If the voltage being held is decreased, inconsistencies occur in images displayed, causing the images not to be sharp. However, since the charged voltage of the transparent electrode 6 leaks through the thin-film transistor section 103, the voltage being held gradually decreases. It is therefore necessary to increase capacitance of the transparent electrode 6, i.e., electrostatic capacitance of the liquid-crystal capacitor section 310 or the capacitor section 105.
In the conventional TFT-type array substrate as shown in FIGS. 30A and 30B, a plurality of address wiring 11 is formed on an insulating substrate 101, on which a gate insulating film 5 is grown and further on which a plurality of data wiring 12 is formed in such a manner that the data wiring 12 and the address wiring 11 cross each other. In each of the picture element areas 102 surrounded by the address wiring 11 and the data wiring 12, the transparent electrode 6 composed of transparent conductive films is formed. Also, in the thin-film transistor section 103 of each of the picture element area 102, the gate 11a connected to the address wiring 11 is adapted to selectively connect either of the data wiring 12 and the transparent electrode 6.
The transparent electrode 6, since its charged voltage is held until it is rewritten by the subsequent data signal, is adapted to constitute a capacitor section 105 allowing electrostatic capacitance to be accumulated between the transparent electrode 6 and the address wiring 11. As shown in FIG. 23B, the capacitor section 105 is so configured that a part of the address wiring 11 faces a part of the transparent electrode 6 through the gate insulating film 5 formed on the address wiring 11 and an upper layer insulating film 8 grown on the gate insulating film 5.
According to the conventional method, in the capacitor section 105, since a dielectric layer between the address wiring 11 and the transparent electrode 6 is thick, electrostatic capacitance per area is small accordingly. Therefore, in order to increase the capacitance, a part of the address wiring 11 is extended to an image section and the area facing the transparent electrode 6 is increased. However, in the case of a light-transmission type liquid-crystal display device in particular, if the part of the address wiring 11 is extended to the image section, an amount of light passing through the image section is decreased, thus resulting in poor lighting on a screen. Accordingly, a new method is needed for increasing the electrostatic capacitance without increasing the area of the capacitor section 105. In Japanese Laid-open Patent Application No. Hei5-2189, a configuration of a conventional picture element area is disclosed as shown in FIG. 31. In this configuration, the gate 11a connected to the address wiring 11 and an auxiliary capacitive common wiring 24 disposed separately from the address wiring 11 are formed on the insulating substrate 101, on which SiO2 films 32 are grown and, at the thin-film transistor section 103, on the SiO2 films 32 are formed a silicon nitride film 33 and an amorphous silicon film 34. After that, on the first SiO2 films 32 is formed the transparent electrode 6 composed of ITO, on which the silicon nitride film 33 is grown. At this point, the silicon nitride film 35 is left on the transparent electrode 6. Next, a source electrode 36a and a drain electrode 36b to be connected via a through hole to the transparent electrode 6 are formed with a second metal material such as Cr/Al and the like and an upper auxiliary electrode 36c to be connected via a through hole to the auxiliary capacitive common wiring 24 is formed with the same second metal materials as well.
According to configurations described above, since electrostatic capacitance is generated in parallel between the transparent electrode 6 and the auxiliary capacitive common wiring 24 and between the transparent electrode 6 and the upper auxiliary electrode 36c, it becomes possible to increase the area effectiveness of the capacitor section 105. However, such conventional technologies have various problems as described below. That is, since configurations of the liquid-crystal display device as disclosed above are more complex than those of conventional ones and its manufacturing processes increase greatly in number, thus resulting in poor productivity of the liquid-crystal display device. Since the source electrode 36a, drain electrode 36b and upper auxiliary electrode 36c, which are all composed of metal materials such as Cr/Al or the like, are exposed on the surface of the TFT array substrate, while application of polyimide and treatment of orientation are being performed at next processes or while the device is being stored before next processes start, such malfunctions as adsorption of water into the metal surface and/or dissolution of the upper auxiliary electrode 36c or of the electrode of the thin-film transistor section 103 occur, thus presenting a problem in reliability of the liquid-crystal display device. Furthermore, since the metal film constituting the upper auxiliary electrode 36c is used as the data wiring, its thickness should be large in order to lower the wiring resistance, i.e., the thickness of the metal film is 10 times larger than that of the transparent electrode 6 (made of ITO). Therefore, if the upper auxiliary electrode 36c is exposed, big steps occur on the surface, causing a loss of flatness in the polyimide film formed on the surface of the TFT array substrate. Because of this, at the area where these steps occur, the image quality is degraded. Additionally, there are other problems in that, as the metal film has a poor wettability to polyimide, voids occur between the data wiring/upper auxiliary electrode 36c and the polyimide film or the data wiring/upper auxiliary electrode 36c peel from the polyimide film.
A method for forming the TFT array substrate with the metal film being unexposed is disclosed in Japanese Laid-open Patent Application No. 10-48664 as shown in FIG. 32, which shows a cross-sectional view of a picture element area in a liquid-crystal display device and in FIG. 33, which shows a cross-sectional view of the picture element area taken on line J-K in FIG. 32. In FIGS. 32 and 33, address wiring 11 and an auxiliary capacitive common electrode 41 are formed on an insulating substrate 101, on which the gate insulating film 5 is formed. On the gate insulating film 5, in the thin-film transistor section 103, a source electrode 4 and a drain electrode 3 are formed. At a capacitor section 105, by using a same metal film, a storage electrode 42 is formed, on which the upper layer insulating film 8 to cover the thin-film transistor section 103, an image section 104 and the capacitor section 105 are grown. Next, in the image section 104 and the capacitor section 105, the transparent electrode 6 is formed. The transparent electrode 6 is connected via a through hole 8a to the source electrode 4 and via a through hole 8b to the storage electrode 42.
According to the method described above, since a surface of the TFT array substrate 300 is covered with the transparent electrode 6 composed of ITO and the upper layer insulating film 8, both of which can be stable in processing of a liquid-crystal system, the problem caused by the exposure of the metal film is solved. However, because the capacitor section 105 formed by the above configurations is composed of the auxiliary capacitive common electrode 41, gate insulating film 5 and storage electrode 42, a thickness of a dielectric layer and a dielectric constant are limited if desired electric characteristics of the thin-film transistor section 103 must be obtained, and an area of the electrode is increased if electrostatic capacity must be increased. As a result, an effective aperture ratio of the image section 104 is decreased and the display image is poorly lit. If the intensity of a back light is raised to make the display image well lit, power consumption is increased as a result. If a screen having larger numbers of picture elements must be obtained in particular, since an area per one picture element is made smaller, the reduction of the aperture ratio becomes remarkable.